PICKLED RED ONION AND TOMATO SALSA:.,Cut each onion in half, slice very finely and place in a bowl. Sprinkle with 1 tablespoon salt and let rest for about 10 minutes.,Cover the onions with lukewarm water and let rest for another 10 minutes.,Rinse and drain the onions.,Add the lime juice and the oil, place in the fridge until about 30 minutes before serving.,Cut the tomatoes in half and slice very finely.,Mix the onions, sliced tomatoes and cilantro, taste and add salt if necessary.,FISH SOUP:.,______,Add the water and cilantro springs, bring to a boil.,Add the tuna and cook for until the tuna is fully cooked, about 15 minutes.,Drain the tuna and keep the broth to cook the yuca.,Separate or break the tuna into small to medium size pieces.,Bring the tuna broth to a boil and add the yucas, cook until tender but firm, about 30-40 minutes for the frozen yuca.,Take the yuca from the broth, remove the strings and cut into bite size chunks.,Add the yuca chunks and tuna pieces to the broth, taste and add salt if needed.,Re-warm the soup if necessary and serve topped with a good amount of pickled onion and tomato salsa. If desired, can also serve with avocado slices and extra lime slices.